Inclusions comprised of alpha-synuclein (a-syn), i.e. Lewy bodies (LBs) and Lewy neurites (LNs), define Parkinson's disease (PD), PD with dementia (PDD), and dementia with Lewy Bodies (DLB). DLB manifests with neuropathology typical of Alzheimer's disease (AD) in addition to LBs/LNs and given that LBs/LNs are present in ~50% of AD brains, PD, PDD, DLB, and AD may represent overlapping neurodegenerative disorders. However, the precise relationship between the clinical phenotypes and each class of central nervous system (CNS) lesions as well as their pathogenic mechanisms are poorly understood. We hypothesize that the progression of DLB and that of PD to PDD result from selective vulnerability of neurons in different brain regions and the cell-to-cell transmission of distinct pathological a-syn species (strains) that differentially promote a-syn and AD pathology in a disease- and brain region-specific manner. To test this hypothesis, we developed a unique neuron-based model of LBs/ LNs to study concomitant a-syn and AD pathology. Briefly, the application of synthetic human a-syn pre-formed fibrils (PFFs) to primary neurons from non-transgenic (non-Tg) mice seeds recruitment and conversion of soluble endogenous mouse a-syn into insoluble LB/LN-like inclusions with deleterious biological and functional consequences. Notably, endogenous mouse tau also is recruited into insoluble tau aggregates in a strain-dependent manner. This model opens up exciting new opportunities for elucidating mechanisms underlying the transmission of pathological a-syn and the role of various a-syn strains in the induction of a-syn and AD pathology during the progression of PD, PDD and DLB. Biochemical, biophysical and cell biological approaches will be used to determine if and how individual synthetic a-syn strains differ in their capacity to recruit endogenous a-syn into LBs/LNs and promote AD-like tau pathology. We will also verify the presence of a-syn strains in authentic LB/LN enriched preparations generated from PD, PDD and DLB brains characterized genetically and neuropathologically through Core C from patients followed by Core B and studied in Projects 1 and II to determine selective vulnerability and a-syn strains. Thus the Aims of Project IV are to: 1) Test the hypothesis that neurons from different CNS regions are selectively vulnerable to develop either LBs/LNs alone or LBs/LNs with AD-like tau pathology and altered levels of secreted Abeta In response to treatment with distinct a-syn PFF strains; 2) Generate and characterize synthetic a-syn PFFs strains that differentially modulate the LBs/LNs and AD pathology; 3) Determine if enriched LBs/LNs fractions isolated from different regions of PD/PDD/DLB brains, will differentially seed and cross-seed the recruitment of endogenous a-syn and tau into insoluble aggregates in primary neurons, thus reflecting strain-like properties; 4) Collaborate with Project IV to identify anti-a-syn monoclonal antibodies that block a-syn transmission in neuron-based synucleinopathy models to be used for immunotherapy in a-syn Tg mice of Project IV.